1. Field of the Invention
This invention relates to an image scanning system, and more particularly to improvements in the control of the signal integration time of image sensing means in an image scanning system using signal integration type image sensing means.
2. Description of the Prior Art
In recent years, a one-dimensional or two-dimensional signal integration type image sensing means such as a charge-coupled device (CCD) has become used in various fields. Such an image sensing means produces an output linearly proportional to the intensity of incident light and therefore its dynamic range is narrow. To cause the image sensing means to respond to a variation in the intensity of a wide range of incident light, a method has been proposed in which an upper limit reference level determined by the saturation level of the output of the sensing means and a lower limit reference level determined with noise such as dark current taken into account are preset, the output level of the sensing means relative to these reference levels is determined by level determination means and the signal integration time of the sensing means is stepwise changed by integration time control means so that the output level of the sensing means is within a predetermined range of levels, thereby enlarging the dynamic range.
Heretofore, design has been made such that the controllable longest integration time or the controllable shortest integration time or the medium integration time is preselected as an initial integration time set by the integration time control means. However, if this has been done, for example, in a system wherein the controllable shortest integration time is set as the initial integration time, where the intensity of the light incident on the sensing means is low, a considerable time has been required until an output of appropriate level is obtained from the sensing means by successively changing the integration time to prolong it by the cooperation of level determination means and integration time control means. Conversely, in a system wherein the controllable longest integration time is set as the initial integration time, a similar demerit has occurred where the intensity of the incident light is high. On the other hand, in a system wherein a medium suitable integration time is set as the initial integration time, such inconvenience is alleviated to some extent, but where a variation in intensity of light over a very wide range is the object, such system is still insufficient and a similar demerit has occurred when the intensity of the incident light has been very high or very low.